Code-division multiple access (CDMA) radio systems are well known. See, generally, CDMA (Cellular Mobile Communicaitions and Network Secrity, Dr. Man Young Rhee, Prentice Hall 1998, ISBN 0-13-598418-1, and standard TIA/EIA/IS-95, hereinafter "IS-95".
In a typical CDMA system, a base station services a plurality of mobile stations in a predetermined geographical area, which in turn is divided into sectors. The sectors are arranged radially from the base station, which is nominally centrally located within the predetermined geographical area. The base station has a plurality of antennas, several (typically two) being deployed to service each sector. The antennas within each sector typically comprise a main antenna and a "diversity" antenna.
FIG. 1 depicts such a base station configuration. It is assumed for purposes of illustration that the base station serves three sectors (denoted a, b, and c) and that there are two antennas within each sector: a main antenna (a1, for example) and a diversity antenna (a2, for example). This is a typical configuration, but those skilled in the art realize that other numbers of sectors and other numbers of antennas per sector may be employed. Those skilled in the art also realize that although an antenna is nominally associated with a particular sector, its coverage may overlap into adjacent sectors.
FIG. 2 shows that within a base station a duplexer 201 is connected to the main antenna to connect it to the output of the power amplifier for transmission to mobile stations ("forward link") and to an RF front end for reception from mobile stations ("reverse link"). Each diversity antenna is connected through a preselector filter 202 to an RF front end for reverse-link reception, and is not used in the present example for forward-link transmission.
FIG. 3 shows at a high level that after each RF front end converts the signal from the antenna to baseband an analog-to-digital converter is provided to convert the signal to a digital stream, which is sent to both the searcher of FIG. 4 and the rake receiver of FIG. 5, both to be described in further detail below.
FIG. 8 depicts at a high level the prior-art functions of a searcher. The signal received from the mobile station is spectrum-spread according to pseudonoise ("PN") codes, and the base station's receiver must despread it according to those same codes. The base station has code generators comparable to those "local replicas" of the spreading codes, but they must be synchronized with the codes embedded in the signal, and the signal has undergone an unknown amount of transmission delay. It is the primary function of the searcher to determine the amount of transmission delay in order to synchronize the local replicas with the received signal. As FIG. 8 shows the received signal is correlated against the PN codes as delayed by varying amounts and the correlation results are accumulated. A correlation result significantly greater than the noise floor indicates that the transmission delay equals the delay of the PN code associated with that correlator.
The searcher often finds several correlation results that arc significantly greater than the noise floor; this indicates reception of multipath components of the signal. The relative delays among the components can be determined according to the PN-code delay associated with the correlator that produced each significantly greater correlation result. Therefore, CDMA base stations employ "rake receivers" to demodulate the received signal, evaluating each of the multipath components according to its delay and summing the results.
FIG. 5 depicts a rake receiver; the prior-art aspects of the one depicted include multiple digital demodulators 530, each preceded by a delay circuit 520 and followed by a weighing circuit 550. (Each such path is known as a "finger" of the rake receiver.) The delay circuits are set according the relative delays among the multipath components found by the searcher, and the outputs are summed by summer 560, thus improving the amount of intelligence recovered from each mobile station's signal. Each digital demodulator despreads the signal according to the aforementioned local replicas of the PN codes (delayed by the amount determined by the searcher), and undoes other modulations that may have been applied by the mobile station's transmitter, such as Walsh-code modulation. Long-Code spreading, etc., as known to those in the art.
Other base stations exist to service other geographical areas. The coverage areas of base stations typically overlap so that a roaming mobile station is nearly always able to communicate with some base station.
A mobile station may change his geographical position while he has a call in progress. He might move from one sector to another of the same base station, or he might move from the coverage area of one base station into that of another base station. Procedures known in the art as "handoff" procedures govern the changes in sectors or base stations as a mobile station moves around.
A mobile station is said to undergo "hard handoff" when he is handed off from one sector to another operating on a different frequency, whether on the same or another base station. He undergoes "soft handoff" when he moves from one sector into another operating on the same frequency, but on a different base station. He undergoes "softer handoff" when handed off from one sector to another operating on the same frequency on the same base station.
Under soft or softer handoff a mobile station begins communicating with the "new" antenna before he loses communication with the "old" one, and can for a time communicate with both of them.
Under hard handoff a mobile station must be instructed to switch to a new frequency band and reinitiate communication. The smooth transition of soft or softer handoff is thus not provided.
A mobile station may monitor the base station's pilot channel as transmitted by one or several base-station antennas according to a "neighbour list" (known in the art, broadcast from the base station to the mobile station). Since transmission of the pilot channel is according to a different PN code offset on each antenna, the neighbour list is essentially a set of PN offsets which the mobile is to scan. (This is in addition to a call the user may have in progress on the traffic channel.) In practice the mobile station might potentially be able to receive the pilot channel from antennas that are not on the neighbour list, or it might not be able to obtain quality reception from antennas that arc on the neighbour list.
Upon determining that the signal strength from an antenna in an additional sector exceeds a predetermined threshold value, the mobile station informs the base station (and through the base station, the base-station controller BSC)) of that fact. In response, an additional channel is set up on the new sector. The mobile station is then able to communicate with several sector antennas, which may be on different base stations, simultaneously. If the mobile station is in communication with several antennas on the same base station, it is said to be in the softer handoff state.
Those skilled in the art of CDMA communications recognize that a base station operates under a "power budget": a base station services a large number of mobile stations, but must do so within an available total amount of power. Accordingly, transmission to each mobile station is carried out with the lowest practical power so that servicing of all the mobile stations can be accomplished within the power budget. CDMA systems therefore provide for increasing or decreasing power dynamically to facilitate maintaining the lowest practical power on each transmission.
The conventional scheme for softer handoff, which relies on assessment by and request from a mobile station, is inherently slow. This means that for a relatively long time communication with a mobile station is taking place over the "old" antenna, which is not as effective for reaching the mobile's present location as the "new" antenna, and which must therefore be operated at higher power, which is detrimental to the power budget.
Accordingly, there exists a need for a CDMA base station to assess the propagation over the paths associated with each of its antennas for each of the current mobile stations, and for the CDMA base station to determine the optimum antenna configuration for transmission to each of the mobile stations.
It is thus an object of the present invention to provide a CDMA base station that assesses quality of propagation to each mobile station through each antenna.
It is a further object of the present invention to provide a CDMA base station that adjusts selection of antennas for transmission to each mobile station so as to optimize transmission to each mobile station.
It is a further object of the present invention to provide a CDMA base station that minimizes power output for transmission to mobile stations.